Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventcal - Facebook events filtered for your calendar
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Facebook Local - Discover places to go and events happening near you
Bigtent - BigTent is a web-based organization and communication portal for groups.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.